Liquid development method and materials

ABSTRACT

Developed images with substantially no fogging in the background areas or streaking in the image areas are obtained in a liquid development system employing a liquid developer comprising an insulating liquid vehicle having dispersed therein charged toner particles and charged extender body pigment particles selected from the group consisting of calcium carbonate, aluminum hydroxide, barium sulfate, aluminum oxide, talc, silica, calcium silicate, magnesium carbonate, magnesium oxide and mixtures thereof.

United States Patent Fukushima et a1.

LIQUID DEVELOPMENT METHOD AND MATERIALS Inventors: Osamu Fukushima;Masamichi Sato;

Yasuo Tamai, all of Asaka, Japan Assignee: Xerox Corporation, Stamford,

Conn.

Filed: Oct. 19, 1972 Appl. No: 298,884

Related US. Application Data Division of Ser. No. 94,967, Dec. 3, 1970,abandoned.

Foreign Application Priority Data [111 3,820,986 1 June 28, 1974 [56]References Cited UNITED STATES PATENTS 2,877,133 3/1959 Mayer 252/6212,890,174 6/1959 V Mayer 252/621 2,899,335 8/1959 Straughan 117/373,301,698 l/1967 Fauser 117/37 Primary Examiner-Ronald H. SmithAssistant Examiner.1ohn L. Goodrow [5 7] ABSTRACT Developed images withsubstantially no fogging in the background areas or streaking in theimage areas are obtained in a liquid development system employing aliquid developer comprising an insulating liquid vehicle havingdispersed therein charged toner particles and charged extender bodypigment particles selected from the group consisting ofcalcium'carbonate, aluminum hydroxide, barium sulfate, aluminum oxide,talc, silica, calcium silicate, magnesium carbonate, magnesium oxide andmixtures thereof.

V 7 Claims, N0 Drawings 1 r LIQUID DEVELOPMENT METHOD AND MATERIALSBACKGROUND OFTHE INVENTION surface of photoconductor material byelectrostatic. means is well known. The. basic electrostatographic.

process astaught by C. F. Carlson inU.S. Pat. No. 2,297,691 involvesplacing .a uniform electrostatic charge on. a photoconductive insulatinglayer exposing the layer to .a lightand shadow image to dissipate thechargeon theareas of the layer exposed to the light and developing theresultingelectrostatic latent image by depositing on the image a finelydivided electroscopic marking material referred to in the art as.toner.The

toner will normally be attracted to those areas of the layer whichretain a charge thereby forming a toner image corresponding to theelectrostatic latent image. The powder image may then be transferred toa support surface such aspaper and permanently affixed to the support byany suitablemeans such as. heat fixing or solvent fixing.Alternatively,the powder image may be fixed to the photoconductive layer ifelimination of the powdertransfer step is desired. In addition, insteadof latent image formation by uniform charging and followed by imagewiseexposure, the latent image may be formed by directly charging the layerin image configuration. Other methods are known for applyingelectroscopic particles to the imaging surface. Included withinthisgroup are the.cascade development technique disclosed byE: N; Wisein U.S.Pat. No. 2,618,552; the

powder cloud development technique disclosed by C. F. Carlson in US.Pat; No. 2,221,776; and the mag netic brush process disclosed forexample, in US. Pat. No. 2,874,063.

Development of an electrostatic latent image may also be achieved withliquid rather than dry developer materials, In conventionalliquiddevelopment, more commonly referred to aselectrophoretic.development, an insulating liquidvehicle having finelydivided solid material dispersedtherein contacts the imaging surface inboth charged and uncharged areas. Under the influence of the electricfield associated with a charged image pattern the suspended particlesmigrate toward the charged portions of the imaging surface separatingout of the insulating liquid. This eletrophoreticmigration of chargedparticles results in the deposition of the charged particles on theimaging surface in image configuration. Electrophoretic development ofan electrostatic latent image may, for example, be obtained by pouringthe developer over the image bearing surface, by immersingthe imagingsurface in a pool of the developer or by presenting the liquid developeron a smooth surface roller and moving the roller against the imagingsurface. The liquid development technique has been shown to providedeveloped images of excellent quality and to provide particularadvantages over other development methods in offering ease in handling.Liquiddeveloprnent systems also capable of providing high developmentspeed, the development speed of commercial embodiments having recentlyreached a level I of as high as about 10 centimeters per second.However, with the currently available liquid development systems, thisdevelopment speed is practical only for line copy since the developmentof continuous tone or halftone images generally requires a much slowerspeed.

The liquid developers generally employed in these electrophotographicimaging processes comprises fine electrically charged particlessuspended in an electrically insulating liquid. In addition to thecharged particles which are generally referred to as toner, it is commonto disperse or dissolve a charge controlling agent to regulate theelectric charge on toner particles and a dispersing agent to obtain astable dispersion.

A typical liquid developer employed in the production of blackdevelopedimages comprises carbon black particles suspended in a liquidsuch as the highly insulating nonpolar organic solvents includingmineral oil, benzene, heptane, cyclohexane and decylene. Typically,charge controlling agents including various res ins, varnishes,nondrying oils and wetting agents may be added to provide the necessarycontrol of charge on the toner particles. While capable of formingsatisfactory images, these liquid developers exhibit variousshortcomings when employed in automatic machine configurations.

Specifically, difficulties are frequently encountered due tothephenomenon of fogging. Fogging occurs when, for example, aphotoconductive insulating layer is charged and exposed to a light andshadow pattern. In the background or nonimage areas, during exposure,light renders the photoconductive layer conductive and dissipates thecharge. However, in these background or nonimage areas when subjectingthe photoconductive insulating layer to even the strongest irradiation,a small electric charge persists in the nonimage or background areas.This small charge attracts a small amount of toner giving rise tofogging in the background areas of the developed print.

In addition, prior liquid developers may result in the formation ofstreaks on the image. Thus, when bringing an electrostatographic imagingsurface bearing an electrostatic latent image into contact with liquiddevel oper, a relative speed component is present between imagingsurface and the liquid developer. Streaks are formed on the imagingsurface in the high density portions of the image along the direction ofmovement of developer relative to the imaging surface.

Furthermore, prior development systems employing a roller developerdispensing device, have resulted in the formation of apparatus stainsdue to the friction between the rollers and the surface holding theelectrostatic latent image. For example, in processes in which theliquid developer is supplied between the surface bearing theelectrostatic latent image and a metal roller which functions as thedeveloping electrode and is rolled across the imaging surface, or inimaging systems in which anelectrophotographic material passes between apair of pinch rollers while the liquid developer is supplied between theroller and the surface bearing the latent image staining of theapparatus may occur. This occurs since the metal rollers are kept indirect contact with the imaging surface, and any minute projectingportions present on the imaging surface are scraped by the metal rollerpermitting toner to be collected thereby fonning stains.

Furthermore, the prior liquid developers are incapable of producingimage tone such as that obtainable in silver halide photographic paper.

SUMMARY OF THE lNVENTlON It is therefore an object of this invention toprovide a liquid development system which overcomes the above noteddeficiencies.

It is another object of this invention to provide a liquid developerwhich produces high density, nonstreaky developed images.

It is another object of this invention to provide a liquid developmentsystem which produces developed images with substantially no fogging.

It is another object of this invention to provide a liquid developmentsystem which is capable of producing fine grain smooth appearancedeveloped images.

It is another object-of this invention to provide a liquid developmentsystem capable of producing developed images having a tone resemblingthe appearance obtained with silver halide photographic paper.

The above objects and others are accomplished, generally speaking, byproviding an electrostatographic imaging system of the liquiddevelopment type wherein a liquid developer comprising an insulatingliquid vehicle, charged colored particles and extender body pigmentparticles charged to the same polarity as the charged colored particlesis employed. In addition, if necessary, charge controlling agents,dispersion stabilizing agents, fixing agents and other well knownmaterials may be suspended or dissolved in the liquid developer.Alternatively, the charge control agents, the dispersion stabilizingagents and fixing agents may be coated on the individual chargedparticles.

Any suitable extender body pigment particle may be employed in thepractice of the present invention. By the term extender body pigment isintended to define that group of finely divided materials which willprovide a color the same as or similar to the background areas of thefinal print surface such as either a photosensitive paper such asElectrofax paper or ordinary paper. In the most practical of operations,therefore,

the body pigment particles will be substantially white since mostcommercial imaging processes produce prints of black image areas onwhite background areas. It is, however, to be understood that ifbackground areas of other colors are desired, appropriately coloredpigments of the background areas may also be employed. Typically, whenemploying body pigment particles in an electrostatographic imagingsystem providing finished copy with white background and dark colorimage areas, pigments with refractive indices not greater than 1.75 maybe employed. Typical specific materials include calcium carbonate,aluminum hydroxide, barium sulfate, aluminum oxide, talc, silica,calcium silicate, magnesium carbonate and magnesium oxide. It isessential that the body pigment particles do not either discolor ordecompose the toner particles.

The body pigment particles employed in the practice of the presentinvention may be of any suitable size. Typically, the body pigmentparticles are within the range of from several tens of times to severaltenths of g the diameter of the color'charged or toner particles. Toprovide uniformity of suspension in the liquid developer and deposition,it is preferred, however, to maintain the size of the body pigmentparticles within a range of several times larger to several timessmaller than the toner particles. Typically, the extender pigment is ofa particle size of from about 0.01 to about 5 microns. The tonersemployed in the practice of this invention may be of any suitable size.Typically, the toner particles do not have an average particle sizeexceeding about 1 micron or less than about 0.01 micron. Thus, theextender body pigment particles will generally be of about the same sizeas toner particles. Generally the number of extender body pigmentparticles may be employed in an amount within the range of from aboutone tenth to about 10 times the amount of the charged toner particlespresent in the liquid developer. However, to avoid the deposition ofundue quantities of extender body pigment particles in the image areas,it is generally preferred that the extender body pigment particles bepresent in an amount within the range of from about one quarter to abouttwice the amount of the charged toner particles. Optimum image densityin the image areas together with substantially complete reduction offogging in the nonimage or background areas, it is generally achievedwhen the number of extender body pigment particles is about the sameamount as that of the charged toner'particle.

The liquid developer of the present invention may be prepared in anysuitable manner. Typically, the liquid developer may be prepared bymixing the several constitu ents of the developer together. It may alsobe prepared by mixing a first liquid portion containing the tonerparticles with an insulating liquid and a second portion containing thebody pigment particles. Alternatively, a paste containing toner and bodypigment particles may be dispersed within the insulating liquid.

From the above description of the invention, the choice of specificmaterials and operating conditions is deemed to be well within the scopeof those skilled in the art and therefore the scope of the invention isnot limited by the hereinabove mentioned illustrative materials. Theextender body pigment particles may be employed with any suitable liquiddeveloper. Typical liquid developers contain liquids of relatively highinsulating value generally having a volume resistivity greater thanabout 10 ohm-cm so as not to effect the electrostatic charge pattern onthe insulating layer and low dielectric constants of less than about3.4. Typical specific vehicles include hydrocarbons such as benzene,xylene, hexane, naptha, kerosene, halogenated hydrocarbons such ascarbon tetrachloride, trichloroethylene and chloroform. Typical chargedtoner particles which may be employed with the liquid developer include,among others, charcoal, carbon black, magnesium oxide, lithopone,cadmium yellow, chrome yellow, cobalt blue, cadmium red, burnt siena,Hansa yellow, rose bengal and phthalocyanine. Typically, the chargedtoner particles are present in the liquid developer in an amount of fromabout 2 to about 20 grams per liter, and are conventionally dispersedand suspended in the liquid by stirring or agitation. Where a highlyuniform stable suspension is desired, this suspension may be passedthrough a colloid mill. The liquid developers according to the presentinvention may be employed to develop electrostatic charge patternspresent on any suitable imaging surface. Basically, any material capableof holding a charge pattern may be employed. Typical materials includedielectric layers, xeroprinting masters and photoconductors. Aparticularly preferred material for use in automatic copying machines isa photosensitive paper comprising photoconductive pigment particles inan insulating binding layer. Typically, this paper comprises zinc oxidephotoconductive particles present in'an insulating binding layer whichis overcoated on a paper substrate. The choice of particular imagingmember and particular development tech nique may be readily determinedby one skilled in the art. For example, the photosensitive paperdescribed above may be substituted with photoconductor materials madefrom cadmium sulfide, zinc sulfide, zinc selenide, cadmium selenide,titanium dioxide, phthalocyanine and polyvinyl carbazole. As previouslydiscussed, the liquid developers according to the present invention mayalso contain dispersed in the insulating liquid vehicle charge controlagents and suspending agents for their well known functions. Theselection of the appropriate materials to perform these well knownfunctions may be readily made by the artisan.

When employing a liquid developer comprising toner particles which arepositively charged,the extender body pigment particles must also bepositively charged in the insulating liquid. The use of calciumcarbonate and aluminum hydroxide as the extender body pigment particlesis particularly preferred when positively charged particles are to beemployed since they become positively charged merely by being suspendedin the insulating liquid. In order to disperse these minute particles toprovide a stable suspension in the insulating liquid a resin may also beemployed. Typically, from about 0.5 to about 50 parts by weight of theresin per part of the extender pigment may be employed. A portion ofthis resin appears to be strongly absorbed on the surface of the calciumcarbonate or the aluminum hydroxide and the charge on these particlesmay be further stabilized depending on the specific resin employed.Typically, from about 2 percent to about 30 percent by weight ofthetotal resin is absorbed on the surface of the extender pigment. Aparticularly preferred resin providing maximum stabilization of chargeand suspension is a rosin modified phenol formaldehyde resin heated withlinseed oil.

While the above explanation has been made principally with regard totoner particles carrying a positive charge, it is to be understood thatthe present invention also encompasses liquid developers containingtoner particles which have been provided with a negative charge. Liquiddevelopers containing negatively charged toner particles may be employed,to develop positively charged image patterns. They may also be used asreversal developers. In both instances, the liquid developers may haveextender body pigments added which are capable of being negativelycharged. It has been observed that the liquid developer will exhibit thebehavior of a negative toner when the amount of negatively charged tonerparticles present in the liquid developer is larger than that of thepositively charged bodypigment particles. Conversely, the developer willexhibit the behavior of a positive toner when the amount of positivelycharged extender pigment is greater than the negatively charged tonerparticles.

6 pigment'particles with a negative charge by coating the particles witha thin layer of a resin which is soluble in the carrier liquid. Anysuitable resin may be employed for this purpose. Typical materialsinclude polyvinyl chloride and copolymers containing vinyl chloride andnitrocellulose.

The liquid developer of the present invention when employed as anelectrophoretic liquid developer is capable of producing images whichare essentially, completely free from background fogging. In addition,the developer of this invention provides images of clear tone resemblingthe appearance of silver halide photographic paper. Furthermore, theliquid developer ac cording to this invention, provides a very finegrain developed toner image. In addition, the absence of stain resultingfrom friction between the developer applicator roller and the imagingsurface wherein a roller developing method is employed is substantiallycompletely eliminated.

While the exact mechanism and reasons for the improved results are notfully understood at the current time, it is presently believed that thedevelopment of images which are substantially free from backgroundfogging is due to a-preferential deposition of the extender body pigmentin the background portions of the imaging surface which thereby preventdeposition and adherence of the toner in these background portions. Itis further believed that the clear tone resembling the appearance ofsilver halide reproductions is due to the fact that the extender bodypigment particles and the toner particles are deposited together andthereby increase the transparency of the toner image. The fact thatfiner grain toner images are capable of being achieved with the liquiddeveloper of the present invention is believed attributable to the factthat aggregates which consist exclusively of a toner material areprevented since there is a simultaneous deposition of toner and extenderpigment in the image areas which tends to suppress the formation oflarge aggregates which consist solely of colored toner material. Theabsence of staining is further believed to be attributed to the factthat the body pigment particles act as a lubricant or to thepreferential deposition of the body pigment particles on the projectingportions of the imaging surface. The ability to produce images which aresubstantially free from streaks in a liquid development system ofincreased development speed has not been satisfactorily explained. Ithas been established, however, that with comparison of liquid developeraccording to the present invention and developers containing resinparticles instead of extender body particles that increased developmentspeed and the above mentioned improved image qualities are achieved.

DESCRIPTION OF PREFERRED EMBODIMENTS The following, nonlimiting examplesfurther define, describe and compare preferred materials, methods andtechniques of the present invention. Example II is presentedforcomparative purposes. Unless otherwise specified, all amountsproportions and percentages are by weight.

EXAMPLE I shadow pattern in conventional manner. The electrostaticlatent image formed on the zinc oxide sheet is developed by passing thesheet through a bath of a liquid developer made according to thefollowing procedure: Paste A is obtained by ball milling the followingcomposition for 8 hours.

Carbon black 4000g (particle size 0.05 0.lp.) Varnish obtained byheating a mixture of SOOOg equal amounts of rosin-modifiedphenolformaldehyde resin and linseed oil Linseed oil lOOOg Paste B isobtained by kneading the following composition on a three roll kneader.

Aluminum hydroxide 2000g (particle size 0.07 0.2;1.)

Varnish obtained by heating a mixture of 4000g rosin-modifiedphenol-formaldehyde resin and linseed oil Linseed oil lOOOg Both paste Aand paste B contain particles having a charge of positive polarity. Theliquid developer is obtained by dispersing 20 grams of paste A and 15grams vof paste B into 10 liters of Decalin, decahydronaphthaleneavailable from E. I. duPont de Nemours and Company. The developed imageon the zinc oxide sheet is substantially free of fogging in thebackground areas, is substantially free of streaks in the image area'sand has a relatively fine grain image area. In addition, the toneachieved is similar to that obtained with silver halide photographicpaper.

EXAMPLE II The procedure of Example I is repeated except that only 20grams of paste A' is dispersed in 10 liters of Decalin. The developedimage on zinc oxide paper has a high fogging level, streaky images and arelatively coarse grain image. In addition, the tone is inferior to thatobtained in the silver halide photographic paper.

EXAMPLE III The procedure of Example I is repeated except that theliquid developer is made by dispersing 20 grams of paste A in 5 litersof Decalin and grams of paste B in 5 liters of Decalin". When mixedtogether in equal amounts at liquid developer is formed which whenemployed to develop an electrostatic latent image formed in the mannerdescribed in Example I produces prints of quality similar to thatobtained in Example 1.

EXAMPLE IV The procedure of Example I is repeated except thatdevelopment of the electrostatic latent image is accomplished with aliquid developer made by blending the following composition in a ballmill for 1 hour.

Phthalocyanine blue Polymerized linseed oil -Continued Linseedoil-modified alkyd resin 500g Aluminum hydroxide 200g Kerosene 300mlForty grams of this paste is then dispersed in 10 liters of kerosenewhile under the influence of ultrasonic wave. The toner has a stablepositive charge in the carrier liquid. When used as a liquid developer,in the manner described in Example I, an image with substantially nofogging or streaking is obtained. In addition, the image is moretransparent than that obtained by means of ordinary electrophotographicprocessing.

EXAMPLE V The procedure of Example I is repeated with a liquid developerprepared by blending the following composition in a ball mill for onehour in the following manner.

Brilliant Carmine 6B 250g Polymerized linseed oil 250g Linseedoil-modified alkyd resin 300g Precipitated barium sulfate 200g Kerosene300ml Thirty grams of this paste are dispersed while under the influenceof ultrasonic wave into a mixture of 8 liters of cyclohexane and 2liters of kerosene to provide a liquid developer capable of providing ared image. The

precipitated barium sulfate is provided with positive charge in thecarrier liquid. Image quality similar to that described in Example IV isachieved.

EXAMPLE VI The procedure of Example I is repeated with the liquiddeveloper prepared by blending the following composition in a ball millfor 1 hour.

Benzidine yellow 40g Varnish (same as in Example I) g Linseed oil 60gAluminum hydroxide 15g Calcium carbonate 15g EXAMPLE v11 Aphotoconductive insulating layer comprising a paper substrate with acoating thereon of zinc oxide photoconductive particles in an insulatingresinous binder is charged and exposed to a negative image. Theelectrostatic latent image thus formed is developed with a liquiddeveloper prepared by blending the following composition in a ball mill.

Calcium carbonate 250g Vinyl chloride-vinyl acetate copolymer l25gVarnish obtained by heating rosin-modified phenol-formaldehyde resin andlinseed oil 400g Ethyl acetate 3000ml Thirty milliliters of the paste isdispersed while under the influence of ultrasonic wave in a carrierliquid of the following composition.

C yclohexane l 500ml Kerosene 300ml Cottonseed oil 200ml toner of thefollowing composition is prepared in the same manner.

Phthalocyaninc green g Polymerized linseed oil 25g Cyclohexane l500mlKerosene 475ml One part by volume of the green developer and one part byvolume of the liquid developer containing the body pigment are mixed toobtain liquid developer, in which the toner has a stable negativecharge. The negative electrostatic latent image formed on zinc oxidephotosensitive paper is developed with this liquid developer to obtain asatisfactory reversal image. Streaking and fogging are barely observedon the developed image.

Although particular embodiments have been set forth using thedevelopment system and technique of this invention, these are merelyintended as illustrations of the present invention. There are othersystems and techniques which may be substituted for those described.Other modifications of the present invention will occur to those skilledin the art upon a reading of the present disclosure which modificationsare intended to be included within the scope of this invention.

What is claimed is:

l. A method of producing developed images having reduced backgroundcomprising forming an electrostatic latent image on an imaging surfaceand developing said imaging surface with a liquid developer comprisingan insulating liquid vehicle having dispersed therein charged tonerparticles and extender body pigment particles selected from the groupconsisting of calcium carbonate, aluminum hydroxide, barium sulfate,aluminum oxide, talc, silica, calcium silicate, magnesium carbonate,magnesium oxide and mixtures thereof, said body pigment particles havinga charge of the same polarity as said charged toner particles wherebysaid electrostatic latent image is developed uniformly with said tonerparticles while simultaneously the extender body pigment particlespreferentially deposit in the background portions of said imagingsurface.

2. The method of claim 1 wherein said extender body pigment particleshave the same color as the background areas of said imaging surface.

3. The method of claim 1 including the step of transferring said liquiddeveloper from said imaging surface to a receiver sheet, said extenderbody pigment particles having the same color as said receiver sheetsurface.

4. The method of claim 1 wherein said imaging surface comprisesphotoconductive zinc oxide particles dispersed in an insulating binderlayer present on a substrate and said extender body pigment particleshave the same color as the surface of said insulating layer.

5. The method of claim 1 wherein said extender body pigment particlesare present in from about one tenth to about ten times the amount of thecharged toner particles.

6. The method of claim 1 wherein said extender body pigment particleshave a particle size of from about 0.01 to about 5 microns and saidtoner particles have a size of from about 0.01 micron to about l micron.

7. The method of claim 1 wherein said extender body pigment particleshave a refractive index less than about 1.75..

photoconductive

2. The method of claim 1 wherein said extender body pigment particleshave the same color as the background areas of said imaging surface. 3.The method of claim 1 including the step of transferring said liquiddeveloper from said imaging surface to a receiver sheet, said extenderbody pigment particles having the same color as said receiver sheetsurface.
 4. The method of claim 1 wherein said imaging surface comprisesphotoconductive zinc oxide particles dispersed in an insulating binderlayer present on a substrate and said extender body pigment particleshave the same color as the surface of said photoconductive insulatinglayer.
 5. The method of claim 1 wherein said extender body pigmentparticles are present in from about one tenth to about ten times theamount of the charged toner particles.
 6. The method of claim 1 whereinsaid extender body pigment particles have a particle size of from about0.01 to about 5 microns and said toner particles have a size of fromabout 0.01 micron to about 1 micron.
 7. The method of claim 1 whereinsaid extender body pigment particles have a refractive index less thanabout 1.75.